Seedless mutant 'Wuzi Ougan' (Citrus suavissima Hort. ex Tanaka 'seedless') and the wild type were compared by iTRAQ-based quantitative proteomics and integratedly analyzed with transcriptome to improve understanding of male sterility.

Background: Bud mutation is a vital method of citrus. 'Wuzi Ougan' (mutant type, MT) as a bud variant of 'Ougan' (wild type, WT) was first found in 1996 and has become popular because of its male sterility and seedless character. Previous analysis of its cytological sections and transcriptome revealed that the abnormal microsporogenesis that occurs before the tetrad stage of anther development might be the result of down-regulated oxidation-reduction biological processes in MT. To reveal the mechanism behind the male sterility in MT at the post-transcriptional stage, proteome profiling and integrative analysis on previously obtained transcriptome and proteome data were performed in two strains.

Results: The proteome profiling was performed by iTRAQ (isobaric Tags for relative and absolute quantitation) analysis and 6201 high-confidence proteins were identified, among which there were 487 differentially expressed proteins (DEPs) in one or more developmental stages of anthers between MT and WT. The main functional subcategories associated with the main category biological process into which the DEPs were classified were sporopollenin biosynthesis process and pollen exine formation. The enriched pathways were phenylpropanoid biosynthesis, flavonoid biosynthesis, and phenylalanine metabolism. Moreover, there were eight pathways linked in terms of being related to phenylpropanoid metabolism. Eighteen important genes related to phenylpropanoid metabolism were also analysized by qRT-PCR (quantitative real time PCR). An integrative analysis of the fold change at the transcript (log2 FPKM ratios) and protein (log1.2 iTRAQ ratios) levels was performed to reveal the consistency of gene expression at transcriptional and proteomic level. In general, the expression of genes and proteins tended to be positively correlated, in which the correlation coefficients were 0.3414 (all genes and all proteins) and 0.5686 (DEPs and according genes).

Conclusion: This study is the first to offer a comprehensive understanding of the gene regulation in 'Wuzi Ougan' and its wild type, especially during the microsporocyte to meiosis stage. Specifically, the involved genes include those in phenylpropanoid biosynthesis, flavonoid biosynthesis, and phenylalanine metabolism, as determined by integrative transcriptome and proteome analysis.